net/mlx5: Add counter information to mlx5 driver documentation

Update rst file to contain general information about statistics counters
for the mlx5 driver. Add specifics about individual counters in list
tables.

Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>